1. Field of the Invention
The invention relates to control systems for navigable craft particularly with regard to flight control systems for aircraft utilizing limited authority actuators.
2. Description of the Prior Art
Full capability automatic flight control systems have been utilized in aircraft to provide short term stabilization as well as long term attitude and path control via full authority actuators of the parallel, series, or combined series/parallel types for the various control axes of the craft. Such systems are heavy, bulky and expensive and hence are normally utilized only in large scale aircraft such as twin rotor or large single rotor helicopters as well as large scale fixed wing aircraft which can readily accommodate the substantial weights of such complex systems. Amongst the heaviest, bulkiest and most expensive components of such systems are the full authority parallel actuators or more limited authority actuators with separate trim actuators as required to exercise control over the aircraft. These actuators are also exceedingly difficult and hence expensive to install.
Limited capability and hence lower cost automatic flight control systems have been and still are utilized in the prior art to exercise limited attitude control. However, such rudimentary automatic systems do not provide long term path control.
Stability augmentation systems (SAS) are also known for providing short term stability augmentation about the various axes of the aircraft, such systems normally utilizing limited authority series actuators that operate in response to short term sensors such as rate gyroscopes or vertical gyroscopes providing derived rate signals. Limited authority series actuators, as is known, are smaller, lighter, less complex and hence less expensive than full authority parallel actuators as described above.
In order to assist the human pilot in his navigation as well as his attitude and path control tasks, aircraft have also been equipped with flight director systems (FDS) that include flight director computers for providing signals to a flight direct instrument that instructs the pilot in a well known manner how to manually maneuver the craft so as to effect the desired control.
As is known, a helicopter is a particularly difficult aircraft to fly. Because of its inherent instability it normally requires two-handed control under all flight conditions and as a result, is exceedingly difficult to fly under instrument flight rule (IFR) conditions because the additional navigation requirements under such conditions (e.g. tuning communication and navigation radios, selecting courses and altitudes, checking charts, etc.) overtax and human pilot's capabilities. In order to relieve the human pilot of the task of stabilizing this unstable aircraft on a short term basis, many helicopters are provided with a stability augmentation system including a limited authority series actuator (short throw hydraulic or electric servo) controlled from rate gyroscopes or derived rates from vertical and directional gyroscopes. To further simplify the human pilot's tasks, particularly his navigational tasks, many helicopters are equipped with flight director systems.
While full authority, full capability automatic flight control systems have been installed in helicopters, these installations have been limited to only the very largest types of craft such as twin rotor or large single rotor helicopters which can easily accommodate the large weights and avionics/airframe cost ratio of a complete AFCS as discussed above. Thus smaller utility type helicopters as well as small fixed wing aircraft have been limited to the performance provided by short term limited authority stabilization systems such as a SAS and the navigation/path control assistance provided by a flight director system. Thus, heretofore, aircraft such as helicopters provided with a combination SAS/FDS were restricted to full time manual flight.